Field of the Invention
The present invention relates to a sheet feeding apparatus having a containing area for feeding sheet and a containing area for replenishing sheet, and an image forming apparatus.
Description of the Related Art
Conventionally, as a sheet feeding apparatus for an image forming apparatus such as a copier or a printer, what is called a tandem sheet feeding apparatus has been proposed so as to enhance convenience during high-volume continuous printing. In this tandem sheet feeding apparatus, a containing area for feeding in which sheets to be fed for image formation are contained, and in addition, a containing area for replenishing is provided adjacent to the containing area for feeding, and when sheets in the containing area for feeding run out, sheets in the containing area for replenishing are transferred to the containing area for feeding. This increases the number of sheets that can be continuously fed and decreases the frequency with which a user replenishes sheet. According to U.S. Pat. No. 6,332,609 a transfer guide is moved to collectively transfer a sheet bundle contained in the containing area for replenishing to the containing area for feeding.
In such a tandem sheet feeding apparatus, high-volume continuous feeding which is an original purpose is accomplished only by properly stacking sheets in the containing area for feeding and the containing area for replenishing. It is thus necessary to take a prevention measure against abnormal stacking of sheets. According to an arrangement proposed in U.S. Pat. No. 6,332,609, an intermediate sensor is placed midway between the containing area for feeding and the containing area for replenishing, and based on a result of detection by the intermediate sensor at a time point when transfer of sheets to the containing area for feeding is completed, abnormal stacking of sheets in the sheet feeding apparatus is detected.
In the sheet feeding apparatus described in U.S. Pat. No. 6,332,609, however, a user may position the transfer guide between the containing area for feeding and the containing area for replenishing by mistake (for example, by manual operation). When sheets in the containing area for feeding run out during sheet-feeding control in that situation, a sheet bundle in the containing area for replenishing cannot be transferred to the containing area for feeding because the transfer guide is abnormally positioned midway between the containing area for feeding and the containing area for replenishing. Thus, a large amount of sheets cannot be continuously fed. Particularly, with the arrangement described in U.S. Pat. No. 6,332,609), abnormal positioning of the transfer guide cannot be noticed unless there is a change in output from the intermediate sensor. Namely, the user cannot notice that sheet replenishment is impossible under the present conditions. This problem arises when, for example, the transfer guide has such a mechanical configuration as to be undetectable by the intermediate sensor.